Varactors are devices whose capacitance varies with applied voltage. Varactors are typically made using MOS capacitors whose depletion region varies with applied voltage, resulting in roughly 3:1 variation in capacitance. Silicon varactors (e.g., solid-state varactors) have poor isolation from the silicon substrate and also are limited in their tuning range.
Typically, solid-state varactors are employed where tunable capacitance is required. However, solid-state varactors provide a very limited tuning range and have a high resistive loss and relatively high power consumption. For example, in a solid state varactor diode, the varactor's capacitance is set by a bias current generated by a sub-circuit that consumes a significant amount of steady state power. Also, the signal current applied to a solid-state varactor may tend to affect the capacitance, thus inducing some measure of error.
To avoid these problems, a MEMS varactor may be used in the place of solid-state varactors. However, known MEMS varactors suffer from a small tuning range (<3:1) due to the “snap down” effect. This effect causes the gap between two plates of the varactor to close abruptly as the electrostatic attraction force provided by a pair of actuation electrodes exceeds the spring restoring force of the MEMS beam. Once spacing is decreased by more than one third, the snap down phenomenon takes effect and the formerly free end of the MEMS beam makes contact with the base of the device. Because of the snap down effect, MEMS varactors have often been employed as bi-stable devices, rather than as true varactors continuously tunable over a full range of capacitances.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.